The present invention relates generally to a method and apparatus for rapid adsorption-desorption CO2 capture.
The capture and sequestration of CO2 from fossil fuel-fired power plant flue gas is an important step in controlling global warming due to fossil plant energy production.
CO2 separation and capture from flue gases of various stationary sources can be described by either post-combustion, pre-combustion, or oxy-combustion configurations. In the post-combustion configuration, CO2 is captured from the flue gas after the fuel is combusted. When air is used as an oxidant, the combustion flue gas is diluted with the nitrogen in the air; thus, the CO2 concentration in the post combustion flue gas is usually low and ranges from 10-15% by volume for a pulverized coal (PC) fired power plant. For each MW of electric generation capacity, a PC boiler produces a flue gas volume of about 3,500 acfm and emits roughly 1 ton of CO2 each hour. Due to the low concentration of CO2 in the flue gas, low operating pressure, and large volume of flue gas to treat, the post-combustion configuration requires larger equipment and, hence, a higher capital cost.
One option available for lowering the capital cost is post-combustion capture using sorbent beds to adsorb the CO2 from the flue gas. However, current sorbent bed technologies require very large beds of granular sorbents, are difficult to regenerate, and require high energy consumption. Based on the operation modes, adsorption processes include temperature swing adsorption (TSA), pressure swing adsorption (PSA), and vacuum swing adsorption (VSA). VSA is a PSA process in nature. Adsorption processes have several process configurations, such as fixed bed, moving bed, fluidized bed, and simulated moving bed (SMB). Most of the TSA and PSA processes employ the fixed bed configuration. FIGS. 1 and 2 show typical TSA and PSA adsorption processes for CO2 separation.
In adsorption processes, gases or vapors can be captured through chemical or physical interaction with a porous solid adsorbent such as zeolite or activated carbon. Gas separation is achieved when certain species are preferentially adsorbed and subsequently regenerated at high purity.
For TSA applications, CO2 is generally adsorbed at temperatures between 10° C. and 60° C. while regeneration is conducted at greater than 100° C. With large beds, it takes a long time to heat up (to regenerate, such as using steam) and cool down (for adsorption, such as using air) due to heat transfer limitations. The steam used for heating also attacks some of the sorbents, especially if they condense and collect on the sorbent surface. During the adsorption cycle, mass transfer and diffusion is a rate limiting step requiring very large beds to adsorb CO2 due to the large quantity of CO2 to be adsorbed and the large granules needed for the beds. In addition, pressure drop across the beds is also a concern.
For example, in a typical TSA implementation for a 500 MW PC power plant, CO2 emission from the power plant is about 500 ton/hr. The following conditions are assumed for the TSA process:    CO2 removal rate from the flue gas: 90%    CO2 working capacity for the adsorbent: 8% (could be significantly lower)    Adsorption/desorption cycle time in TSA: 2 hours    Bed utilization: 90%
According to these assumptions, the total amount of sorbent required is:Total sorbent=500*90%*2*/8%/90%=12,500 (ton).
This results in a large amount of sorbent. Using the design parameters given in Table 1 below, the minimum number of columns needed is:Number of Columns=12500/(π*3.1*3.1*32/4)/0.8=65The total bed volume (65 columns of sorbent material) required is 15, 600 m3.
TABLE 1WorkingSorbentColumncapacity (gCyclepackingdiameterHeightCO2/gtimedensityColumns(m)(m)sorbent)(hour)(ton/m3)requiredTSA3.1320.0820.865
It should be pointed out that the adsorption-desorption cycle time may vary. However, considering a column size of 3.1 m in diameter and 32 m in height the total cycle time (mostly because of heating and cooling the adsorbent during regeneration) of 2 hours for a TSA process is very moderate. Real cycle time may be well above 2 hours and the column number will accordingly increase.